The present invention relates to a wavelength division multiplexer for use in a wavelength division multiplexing (WDM) transmission. Particularly, it relates to a small-sized and low-cost wavelength division multiplexer having little insertion loss and little polarization dependence and having a broad wavelength bandwidth.
In a WDM optical transmission, a wavelength division multiplexer combines optical signals of various wavelengths, the signal consisting of the various wavelengths is transmitted to a transmission path, and a wavelength division demultiplexer separates the optical signal received from the transmission path into respective wavelengths. In the WDM, the quantity of signals which can be transmitted by one transmission path is the product of the bandwidth of one wavelength and the divided number of the wavelength. In one wavelength, the bandwidth which can be modulated is approximately several tens of GHz under the current circumstances. For increasing the quantity of transmission, it is preferred that the divided number is large.
U.S. Pat. No. 5,748,350 (May 5, 1998) disclosed by Jing-Jong Pan et al. discloses an example concerning a wavelength division multiplexer. This publication discloses a method in which wavelengths are individually one by one separated or combined with a dielectric multilayer filter. Since the configuration of the above method is simple, it has advantages in size reduction and cost reduction. On the other hand, it is required to use dielectric multilayer filters of the same number as the number of wavelengths to be separated or combined. When a few wavelengths are separated or combined, no problem occurs. However, when several tens of wavelengths are separated or combined, the number of parts increases and a defect is that to reduce a size or reduce a cost is difficult.
Further, U.S. Pat. No. 6,112,000 (Aug. 29, 2000) disclosed by Ernest Eisenhardt Berbmann discloses a method in which wavelengths are separated or combined with AWG (Arrayed Waveguid Grating) in one lump.
This method uses the principle in which a phase contrast is generated between arrayed waveguides and the phase contrast differs depending upon a wavelength so that angle of diffraction differs. In this method, the waveguide itself is produced by an expensive semiconductor process. Further, the waveguide has a thicker film and a broader area when compared with a LSI so that very high cost is required. In the above principle, further, the speed of light propagating in each waveguide is apt to be affected by a temperature distribution. Accordingly, it is required to control a temperature with high accuracy for controlling the phase contrast between the waveguides. The defect is that its cost becomes higher.
As another example, there is used a method in which light consisting of various wavelengths is separated or combined with a grating in free space in one lump. Similarly to AWG, this method uses the principle that a phase contrast is generated with a grating, the phase contrast differs depending upon a wavelength and accordingly angle of diffraction differs. Since optical parts used are a grating and a lens which can be produced at a low cost and since the path which light passes through exists in air, almost no temperature dispersion occurs in the above principle. Accordingly, the advantage is that it is not required to control a temperature with high accuracy. The grating used in the above method has diffraction efficiency of nearly 100% in transverse magnetic (TM) polarized light having a vibrational direction of an electric field in the direction normal to grooves. However, it has low diffraction efficiency in transverse electric (TE) polarized light having a vibrational direction of an electric field in the direction parallel to the grooves. Therefore, it is required to take measures against the polarization dependence of the grating.
U.S. Pat. No. 5,886,785 (Mar. 23, 1999) disclosed by Herve Lefevre et al. proposes an example concerning a conventional technology of a wavelength division multiplexer improved in such a polarization dependence. According to this proposition, a light beam is separated with a polarization separator into a TM polarization component and a TE polarization component respectively before the beam enters a grating. Then, the TE polarized light is converted into TM polarized light with a xcex/2 plate and the TM polarized light enters the grating.
Due to the use of such a system, even though light enters a wavelength division multiplexer under any polarization conditions, the polarization component entering the grating is TM polarized light alone. Accordingly, the diffraction efficiency in the grating can become almost 100%.
On the other hand, the polarization separator and the xcex/2 plate are used so that the defect is that its cost becomes high.
xe2x80x9cDipak Chowdhurry, IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, Vol.6, No.2, pp233-239 (2000)xe2x80x9d (to be referred to as xe2x80x9cliterature 1xe2x80x9d hereinafter) proposes another example concerning a conventional technology of a wavelength division multiplexer improved in such a polarization dependence. According to the above literature 1, the polarization dependence is improved by using a grating having a several-tenth order of diffraction.
According to xe2x80x9cPrinciple of Opticsxe2x80x9d (translated by Kusakawa et al., published by Tokai University Press, p608, to be referred to as xe2x80x9cliterature 2xe2x80x9d hereinafter), a grating which allows radiation of a specific wavelength to center at a specific diffraction order is called xe2x80x9cblazed gratingxe2x80x9d. Generally, a grating having a remarkably low diffraction order like a several-order diffraction order, in particular a grating having the first diffraction order, is used in many cases. In contrast, a grating having a several-tenth order of diffraction is called Echelette.
According to the literature 1, Echelette shows high diffraction efficiencies of both TM polarized light and TE polarized light. However, even the above grating having a several-tenth order of diffraction does not have diffraction efficiency of nearly 100% concerning the TE polarized light. Further, a grating having such a higher diffraction order has a defect, that is, the higher the diffraction order is, the narrower the reflection bandwidth is.
As described before, concerning a wavelength division multiplexer which separates or combine light consisting of various wavelengths with a grating in free space in one lump, there can be produced a small-sized and low-priced wavelength division multiplexer which combines or separates many wavelengths, such as several tens of wavelengths. However, it is required to cope with the polarization dependence. Accordingly the defect is that a cost increase occurs or deterioration of properties such as an insertion loss, polarization dependence or wavelength bandwidth occurs.
It is an object of the present invention to provide a small-sized and low-cost wavelength division multiplexer having little insertion loss, little polarization dependence and a broad wavelength bandwidth.
According to the present invention, there is provided a monochrometer comprising a grating, in which an incident angle of light entering said grating agrees with an exit angle of a diffracted light exiting said grating, wherein said grating is composed of a plane substrate and linear grooves constructed of two facets, said linear grooves are formed on said plane substrate in regular cycles, the three angles of xcex8a, xcex8b and xcfx86 satisfy the relationships of the following equations 1 and 2 in which xcex8a is the angle that the normal line to one facet of the two facets of each groove forms with the normal line to the surface of said plane substrate, xcex8b is the angle that the normal line to the other facet of the two facets of each groove forms with the normal line to the surface of said plane substrate, and xcfx86 is the incident angle which light entering said grating forms with the normal line to the surface of said plane substrate, and said incident angle xcfx86 satisfies the following equations 3 and 4 in which m is the order of diffraction and each of n1 and n2 is a whole number.
xcex8a=xcfx80/4+xcfx86/2xe2x80x83xe2x80x83(1)
xcex8b=xcfx80/4xe2x88x92xcfx86/2xe2x80x83xe2x80x83(2)
sin xcfx86=m/(2n1+m)xe2x80x83xe2x80x83(3)
sin xcfx86=m/(2n2xe2x88x92m)xe2x80x83xe2x80x83(4)
According to the present invention, there is also provided a wavelength division multiplexer for use in a wavelength division multiplexing (WDM) optical transmission system which conducts communications by sending and receiving a wavelength division multiplexing (WDM) signal obtained by combining optical signals of a plurality of wavelengths, which wavelength division multiplexer comprises light input/output means to a fiber, the monochrometer recited in claim 1, and collective means disposed on an optical path between said light input/output means and the monochrometer.